Before tag libraries, JSPs were difficult to maintain because you were forced to use JavaBeansTM components and scriptlets as the main mechanism for performing tasks. Custom actions, that is, a tag library, alleviate this problem by bringing the benefits of another level of componentization to JSP. A tag library encapsulates recurring tasks so that they can be reused across more than one application. The tags in the JSP tag library fall into three basic groups: context setup tags, validator tags, and normal tags.
Context setup tags
These tags, which start with the prefix obtain, set up the context (storing the container or provider in question into the pageContext).
These tags validate that the provider in context can legally use the tags in the TLD.
These tags serve as wrappers of PAPI, ProviderContext, ContainerProviderContext, and the specific containers in the Sun Java System Portal Server 7.1 software.